Study finds role for parasites in evolution of sex

Jul 06, 2009

What's so great about sex? From an evolutionary perspective, the answer is not as obvious as one might think. An article published in the July issue of the American Naturalist suggests that sex may have evolved in part as a defense against parasites.

Despite its central role in biology, sex is a bit of an evolutionary mystery. Reproducing without sex—like microbes, some plants and even a few reptiles—would seem like a better way to go. Every individual in an asexual species has the ability to reproduce on its own. But in sexual species, two individuals have to combine in order to reproduce one offspring. That gives each generation of asexuals twice the reproductive capacity of sexuals. Why then is sex the dominant strategy when the do-it-yourself approach is so much more efficient?

One hypothesis is that parasites keep asexual organisms from getting too plentiful. When an asexual creature reproduces, it makes clones--exact genetic copies of itself. Since each clone has the same genes, each has the same genetic vulnerabilities to parasites. If a parasite emerges that can exploit those vulnerabilities, it can wipe out the whole population. On the other hand, sexual offspring are genetically unique, often with different parasite vulnerabilities. So a parasite that can destroy some can't necessarily destroy all. That, in theory, should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites.

The scenario works on mathematical models, but there have been few attempts to see if it holds in nature.

Enter Potamopyrgus antipodarum, a snail common in fresh water lakes in New Zealand. What makes these snails interesting is that there are sexual and asexual versions. They provide scientists with an opportunity to compare the two versions side-by-side in nature.

Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl of the University of Washington and Curtis Lively of Indian University, Bloomington began observing several populations of these snails for ten years starting in 1994. They monitored the number of sexuals, the number asexuals, and the rates of parasite infection for both.

The team found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time. As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time. This, the authors say, is exactly the pattern predicted by the parasite hypothesis.

"The rise and fall of these female-only lineages was surprisingly fast and consistent with the prediction of the parasite hypothesis for sex," Jokela said. "These results suggest that sexual reproduction provides an evolutionary advantage in parasite rich environments."

So we may well have to thank parasites—in spite of their nasty reputation—for the joy of sex.

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User comments : 6

This type of thinking, is presuming that sexuals necessarily have some genomic variability, more than what asexuals have. Otherwise, if they have somehow the same range of variability in the species, that is the within-species (but not within-individual) variability. That is, the species might live through its subspecies. No advantage of sexual vs. asexual reproduction, there. Not to mention that, asexuals are told to reproduce massively more than the sexuals.

((Like telling a gun is more lethal than a chemical, if what you would give in the chemical category, is only a tolerable dose of the chemical. That comparison is not fair.))

Furthermore, the species is not only the children of one bacteria. If all of the offsprings of one bacteria are killable (in the niches they have), some other of the same species, might live. Those subspecies which find their niche, keep living on. Statistically, presumable that, the subspecies that is massively reproducing, and some of those find some fitting niche, will live. That keeps the species up, statistically.

Concerning "evolution" vs. creation: Whatever is statistical (having gaps in our knowledge), will boil down to, praying to your favorite god-of-the-gaps. Evolutionists believe in the mutationgod, I believe in Allah, the Creator. (Their mutationgod is strictly a god-of-the-gaps. When data are in, that supports the creation, I witness. Such as the "convergent/parallel-evolutions" (sequential contradictions) obviating the "trees" of evolution.)

To summarize so far, potency/resilience/versatility, and variability are key points. If "sexual" is having more variability or more resilient subspecies, then the key is not sexualness.

Furthermore, bacteria have the capacity of gene graft, too. Actually, they have been known to having that a lot. For humans, that is the gene-therapy notion. Again, I think that is nothing to make new species, but allow the species to actualize its gene-expression flexibilities (toward subspecies that genome is capable of having). I told the spiderman example inhttp://www.I-slam...nsky.htm

On the topic of asexual reproduction; after a long enough period of time, wouldn't whatever species that reproduce this way eventually face diminishing genetic returns? Meaning, if you make enough clones, eventually you reach a point where the copies degenrate exponentially.

Perhaps sexual reproduction is just how most species have over come the dimishing returns?

There is no "return" (feedback) that a subspecies that is gone to some other niche, would give to the species. But the species would be surviving. (In that case, that is a tree-search, with no backtracking.)

Otherwise, if they are living together, then gene graft would do what sexual-reproduction is having. (With their tiny existence, that is massive. A white man, marrying a black woman, might have middle of the two. Bacteria have that through graft. Then, keep replicating.)

The point is that we either have some genetic flexibility or not. (Evolutionists believe in the mutationgod bringing up such variation, I believe in Allah allowing the subspecies to vary.) So far as that genome has the capability of varying (for example, like gene-therapy would successfully hack), is what is happenable.

On the topic of asexual reproduction; after a long enough period of time, wouldn't whatever species that reproduce this way eventually face diminishing genetic returns?

No.

Meaning, if you make enough clones, eventually you reach a point where the copies degenrate exponentially.

There's nothing magic about sex. Mutations occur in sexually reproducing organisms as well as in asexually reproducing organisms. Deliterious mutations are eventually weeded out by competition for resources if they aren't caught early(e.g. miscarriage).

If one person has a gene that allows tolerance to cow milk and another person has a tolerance to plague bacteria, both those genes can end up in a single offspring due to sexual recombination. With clones, those genes will never meet in a single individual but need to be rediscovered in each line of clones. Without sexual reproduction, good genes cannot escape a background of bad genes; it's the total package that is selected against, not just the bad genes.

If one person has a gene that allows tolerance to cow milk and another person has a tolerance to plague bacteria, both those genes can end up in a single offspring due to sexual recombination. With clones, those genes will never meet in a single individual but need to be rediscovered in each line of clones. Without sexual reproduction, good genes cannot escape a background of bad genes; it's the total package that is selected against, not just the bad genes.

Each-line-of-clones might not be encountering all illnesses, any way. Why bother having the tolerance gene, then?

Gene-transfer is available for asexual organisms, too. That is known.

Besides, with that logic of guessing sexual reproduction to have some mighty potential, why would we, humanity, not have the species-wide tolerance to malaria? There is probably few or no such genetic tolerance. (But, vice versa. Mothers transmit immunity to their babies [through milk] -- but that is lastiing only for their first few months, after that they have to develop immunity.) In contrast, a clone is naturally the thing itself, thus presumably, no need to "re-"immune itself, nor any need to get that from elsewhere.

Thus, the stable illnesses are nullifiable by clones, both by keeping the old strengths and by extra transfers of genes. That is, the logic of
(1) "living forever" by multiplying itself, and
(2) the capability to have gene-hacks to confuse invaders

Presumably, the strongest of asexual subspecies,
(1) multiply the most
(2) if there is the need, likely to get the genes from other strong subspecies. Thus, if you would be so optimistic like you are in the case of sexual reproduction, then you would expect that, the two strongest subspecies would swap their strongets genes, because, statistically, they had been the two most widely available subspecies (because of having strengths to fight against various illnesses). But again, I might spoil that optimism a bit, because like we know about Windows, when something is popular, more hackers try to hack that. Likewise, when some subspecies is so widely available, some invader would hone its skills to traget their plentiful resource.

BTW, I forgot the statistics about the thing (if I knew): Is there that guarantee that good genes are dominant? In lots of mental illnesses, having someone in the family, is sufficient, as a risk factor to have the illness (not having that int the other parent, is not the final cure). What makes you so optimistic about the good genes having the high command?

(Let alone the problem that, with sexual reproduction, you have the chance of total loss of the good genes, too. Like, if both of your parents have genes for both brown- & blue-eyes, then half the chance is that, either the blue or the brown genes will not be in the offspring, at all. And if not in the offspring, not selectable.)

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